1. Field of the Invention
The present invention relates to a film winding device for automatically winding a filmstrip after each exposure in a camera, especially, a film winding device which is removably attachable to a lens-fitted photographic film unit.
2. Related Art
Lens-fitted photographic film unit (hereinafter simply referred to as film units) are well known, having a simple photograph-taking mechanism and a ISO 135 type photographic film cassette preloaded therein. Such film units can be purchased nearly everywhere at low prices, making it possible to take photographs anywhere without carrying about heavy and expensive cameras.
The film unit has a unit body which is constituted of a basic portion, a rear cover and a front cover. A photo-taking unit including a taking lens, a shutter mechanism, a film frame counter mechanism, a film winding-stopping mechanism and other elements, is attached to the basic portion. The front cover is removably attached to the front of the basic portion. A roll of unexposed photographic film which is pulled out from a container of the film cassette is placed in a film supply chamber of the basic portion, while the container is placed in a film take-up chamber of the basic portion. A film winding wheel having teeth around its periphery is coaxially engaged with a spool of the container to rotate the spool together.
When the film winding wheel is rotated to wind up the film, the strip of the film extending from the film supply chamber to the film take-up chamber is advanced to rotate a sprocket wheel, such that the sprocket wheel makes one revolution while the filmstrip is advanced one frame amount, and then the film winding wheel and the sprocket wheel is locked to stop rotating. During the one frame advance, the shutter mechanism is set to a cocked position. Upon depression of a shutter button, an exposure is executed and, at the same time, the film winding wheel and the sprocket wheel are unlocked. Thus, the filmstrip is wound up for the next exposure. After the last frame is exposed, the filmstrip is entirely wound into the container by rotating the film winding wheel. Thereafter, the film winding wheel starts idling to notice the user that the film winding is complete. Then, the user forwards the film unit with the exposed filmstrip to a photofinisher. The photofinisher removes the container containing the exposed filmstrip from the film unit, and processes the exposed filmstrip for making prints therefrom in a conventional manner. The consequent prints and the developed filmstrip are returned to the user.
Since the film unit cannot make an exposure unless the film winding wheel is rotated to advance the filmstrip one frame after each exposure and thus cock the shutter mechanism, it is not a few that the photographer forgets to wind up the filmstrip and misses the moment for a good picture. To solve this inconvenience, JPA 5-297441 discloses an automatic film winding device which can be removably attached to the film unit.
The known film winding device is constituted of a shutter release member, an externally operated power switch, a motor, a reduction gear unit, a drive gear, a motor current detection circuit and other elements. The shutter release member is depressed like as a shutter button of the film unit, thereby depressing the shutter button. The drive gear is adapted to gear into the film winding wheel, so that the film winding wheel is rotated by the motor through the reduction gear unit and the drive gear when the power switch is turned ON to activate the motor. When the filmstrip is advanced one frame, the film winding wheel is locked, so that a large load is applied to the motor through the drive gear and the reduction gear unit. As a result, the current flows through the motor jumps up. The motor current detection circuit detects the current through the motor so as to turn the power switch OFF upon the rapid increase of the motor current. Thus, the motor is inactivated when the filmstrip has been advanced by one frame.
However, when the motor stops, a tension is applied to the filmstrip due to the tendency of the sprocket wheel and the film winding wheel toward the film winding direction. Therefore, the moment when the shutter is released and thus the film winding wheel and the sprocket wheel is unlocked, the filmstrip slightly moves in the film winding direction. If the filmstrip is still in motion during an exposure time, the image recorded on the filmstrip is blurred.
Moreover, the known film winding device needs to turn the power switch ON after each shutter release. This is inconvenient, and could be a cause of missing a good shutter chance if the photographer forgets to turn the power switch ON. Also because the reduction gear unit is a simple gear train, the teeth of the reduction gear tend to be damaged due to a large load which is applied during a delay time from the locking of the film winding wheel to the inactivation of the motor by the motor current detection circuit.
Since the motor is stopped when the current therethrough jumps up, the motor continues to be driven after the completion of exposure of the last frame when the film winding wheel will not be locked any more, and hence the motor current does not increase any more. If the film winding unit is detached from the film unit while the motor is being driven, a load would be applied to the film winding wheel and the drive gear, which would damage the teeth of these elements.